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Observation of Enhanced X-Ray Emission from the CTTS AA Tauri During One Transit of an Accretion Funnel flow

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Observation of Enhanced X-Ray Emission from the CTTS AA Tauri During One Transit of an Accretion Funnel flow A&A 475, 607–617 (2007) Astronomy DOI: 10.1051/0004-6361:20078084 & c ESO 2007 Astrophysics Observation of enhanced X-ray emission from the CTTS AA Tauri during one transit of an accretion funnel flow N. Grosso1, J. Bouvier2, T. Montmerle2, M. Fernández3, K. Grankin4, and M. R. Zapatero Osorio5 1 Observatoire astronomique de Strasbourg, Université Louis-Pasteur, CNRS, INSU, 11 rue de l’Université, 67000 Strasbourg, France e-mail: [email protected] 2 Laboratoire d’astrophysique de Grenoble, Université Joseph-Fourier, CNRS, INSU, 414 rue de la Piscine, 38041 Grenoble, France 3 Instituto de Astrofísica de Andalucía, CSIC, Camino Bajo de Huétor 50, 18008 Granada, Spain 4 Ulugh Beg Astronomical Institute of the Uzbek Academy of Sciences, Astronomicheskaya 33, 700052 Tashkent, Uzbekistan 5 Instituto de Astrofísica de Canarias (IAC), vía Láctea s/n, 38205 La Laguna, Tenerife, Spain Received 14 June 2007 / Accepted 27 August 2007 ABSTRACT Context. Classical T Tauri stars are young solar-type stars accreting material from their circumstellar disks. Thanks to a favorable inclination of the system, the classical T Tauri star AA Tau exhibits periodic optical eclipses as the warped inner disk edge occults the stellar photosphere. Aims. We intend to observe the X-ray and UV emission of AA Tau during the optical eclipses with the aim of localizing these emitting regions on the star. Methods. AA Tau was observed for about 5 h per XMM-Newton orbit (2 days) over 8 successive orbits, which covers two optical eclipse periods (8.22 days). The XMM-Newton optical/UV monitor simultaneously provided UV photometry (UVW2 filter at 206 nm) with a ∼15 min sampling rate. Some V-band photometry was also obtained from the ground during this period in order to determine the dates of the eclipses. Results. Two X-ray and UV measurements were secured close to the center of the eclipse (∆V ∼ 1.5 mag). The UV flux is the highest just before the eclipse starts and the lowest towards the end of it. UV flux variations amount to a few 0.1 mag on a timescale of a few hours and up to 1 mag on a timescale of a week, none of which are correlated with the X-ray flux. We model it with a weekly modulation (inner disk eclipse), plus a daily modulation, which suggests a non-steady accretion, but needs a longer observation to be confirmed. No such eclipses are detected in X-rays. Within each 5 h-long observation, AA Tau has a nearly constant X-ray count rate. On a timescale of days to weeks, the X-ray flux varies by a factor of 2−8, except for one measurement where the X-ray count rate was nearly 50 times higher than the minimum observed level even though photoelectric absorption was the highest at this phase, and the plasma temperature reached 60 MK, i.e. a factor of 2−3 higher than in the other observations. This X-ray event, observed close to the center of the optical eclipse, is interpreted as an X-ray flare. Conclusions. We explain the variable column density with the low-density accretion funnel flows blanketing the magnetosphere. The lack of X-ray eclipses indicates that X-ray emitting regions are located at high latitudes. Furthermore, the occurrence of a strong X-ray flare near the center of the optical eclipse suggests that the magnetically active areas are closely associated with the base of the high-density accretion funnel flow. We speculate that the impact of this free-falling accretion flow onto the strong magnetic field of the stellar corona may boost the X-ray emission. Key words. stars: individual: AA Tauri – stars: pre-main sequence – X-rays: stars – accretion, accretion disks 1. Introduction because the X-ray luminosity is proportional to the plasma emis- sion measure, which scales linearly with the plasma volume but T Tauri stars (TTSs), i.e. young (1–10 Myr) solar-type stars, also with the square of the plasma electronic density. That most are conspicuous X-ray emitters. Their high X-ray luminosities 28−31 −1 27 −1 of TTSs’ X-ray emission arises in an active magnetic corona (LX 10 erg s ) compared to the Sun (LX 10 erg s is supported by direct Zeeman measurements on photospheric at solar maximum) and intense flaring activity (up to LX 32−33 −1 spectral lines which indicate surface magnetic fields of a few 10 erg s ) make them appear as extremely active young kilogauss (e.g., Guenther et al. 1999; Johns-Krull et al. 1999, suns in the X-ray domain. The analogy with solar activity has 2004; Johns-Krull 2007; Yang et al. 2005). However, the dy- been quite successful in ascribing the X-ray emission of TTSs namo mechanism producing the magnetic field in these fully to an optically thin, magnetically confined coronal plasma in − convective stars is still being discussed (e.g., Preibisch et al. collisional equilibrium at temperatures of 10 100 MK, emit- 2005; Briggs et al. 2007). ting a thermal bremsstrahlung continuum and emission lines (see, e.g., review by Feigelson & Montmerle 1999). As ob- The solar paradigm, where the X-ray emitting plasma is con- served in the X-ray coronae of active stars (e.g., Ness et al. fined in a magnetic loop with both feet anchored in the stellar 2004), the enhanced X-ray luminosity of TTSs can easily be photosphere, has been questioned in the context of a Classical explained by coronal structures with high plasma density, TTS (CTTS), since it accretes material from its circumstellar disk. Inside a few stellar radii above the CTTS surface, the stel- Figures 2, 3, and 9, and Appendix A are only available in electronic lar magnetic field pressure is higher than the ram pressure of the form via http://www.aanda.org accreting gas. As a result, the stellar magnetosphere truncates the Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20078084 608 N. Grosso et al.: Enhanced X-ray emission from AA Tauri inner accretion disk and controls the accretion flows. The gas is Table 1. Journal of the XMM-Newton observations of AA Tau (PI: mainly accreted from the disk edge to the stellar surface along J. Bouvier). the dominant large-scale stellar magnetic lines, creating accre- tion funnel flows. The free-falling gas hits the stellar surface at Obs. Rev. ObsId Feb. 2003a Exposurea the feet of the accretion funnel flows, where the kinetic energy (d) (h) is dissipated in a shock, producing a hot excess emission (see 1 583 015680201 14.10–14.29 4.7 review on magnetospheric accretion by Bouvier et al. 2007b). 2 584 015680301 16.13–16.33 4.7 3 585 015680501 18.07–18.26 4.7 The X-ray grating spectrometers aboard Chandra and 4 586 015680401 20.02–20.22 4.7 XMM-Newton are able to obtain spectra of the X-ray brightest 5 587 015680601 22.13–22.35 5.4 CTTSs, where the emission line triplets of He-like elements are 6 588 015680701 24.03–24.25 5.3 resolved. This provides a powerful tool for assessing the elec- 7 589 015680801 26.52–26.71 4.6 tronic density of the X-ray emitting plasma (e.g., Porquet et al. 8 590 015680901 28.30–28.50 4.7 2001). In several CTTSs, plasma with high electronic density a 13 −2 The observation beginning, end, and duration is given for MOS1. (ne ∼ 10 cm ) and low temperature (∼3 MK), atypical of stellar coronae, were identified and therefore attributed to ac- cretion shocks (Kastner et al. 2002; Stelzer & Schmitt 2004; distribution to reconcile the high value of the X-ray absorption Schmitt et al. 2005; Robrade & Schmitt 2006; Argiroffi et al. outside the eclipse and the low optical extinction. 2007; Günther et al. 2007; Telleschi et al. 2007), whereas some In Sect. 2, we present the X-ray and UV properties of CTTSs display no such evidence (Audard et al. 2005; Smith et al. AA Tau based on reanalysis of the full data set provided by 2005; Güdel et al. 2007b). these XMM-Newton observations. In particular, we report X-ray During the Chandra Orion Ultradeep Project (COUP, see (pn+MOS1+MOS2) and UV light curves with a time resolution Getman et al. 2005b), where the TTSs of the Orion nebula clus- of ∼15 min. We show that a bright and hot flare was observed ter were monitored almost continuously over ∼13 days with the during the second observation. In Sect. 3, thanks to our ground- Advanced CCD Imaging Spectrometer (ACIS), numerous X-ray based observations and optical OM data, we determine the dates flares were observed. In a few cases, a size of several stellar radii of the optical eclipses. This allow us to compare the UV and was derived for the magnetic loop confining the X-ray emitting X-ray variabilities versus the rotational phase. We show that the plasma, large enough to connect the stellar surface with the edge UV minimum is outside the eclipse and that the (confirmed) vari- of the accretion disk (Favata et al. 2005). ation in the column density are not correlated with the rotational We propose to directly constrain the source and location of phase. In Sect. 4, we propose another origin for this variable col- the X-ray emission in CTTSs by using eclipses. Eclipse map- umn density, and we discuss the origin of the X-ray flare that was ping has been successfully used in binary active stars to recon- observed during an optical eclipse.
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